The invention is described using the example of a bowl-shaped folded box, which is filled with powdered prepared foods, especially baked goods, and which is then closed with a lid. Such a bowl-shaped folded box is known, for example, from German Offenlegungsschrift no. 28 19 708. The packaging bowl shown there consists of cardboard as the base or carrier material. The cardboard is coated with polyethylene, polypropylene, or a similar material, and covered with an appealing colored print on the outside. The package includes a layer of aluminum foil on the inside. Such baking bowls are sold to the consumer filled and sealed with a lid, and the consumer only has to tear off the lid, and place the opened baking bowl into a conventional oven. After baking, the bowl-shaped box can be opened and unfolded so that the finished cake or the like can easily be transferred to other dishes.
The difficulty concerning the known bowl-shaped box is that the materials used must be kept at a rather high temperature for a length of time without a detrimental effect on the food. For example, there are prepared mixes for baked goods which require heating for 40 to 45 minutes at a temperature of 175.degree. C. As a result, the materials from which the folded box is produced must be resistant to such temperatures for the amount of time required. Other prepared foods must be brought to a temperature of approximately 150.degree. C. in an automated convection oven.
Sealing of the baking bowls has been a special source of significant difficulties, especially since the layer of aluminum foil excludes the possibility of heating by microwave, and for this reason the use of different synthetic heat sealable materials and lacquers as the material for coating the cardboard have been tried. The temperature limit finally set for the cooking or baking process is determined by the synthetics used. For example, it has been determined that this temperature limit lies in the range between 100.degree. and 105.degree. C. for polyethylene; between 145.degree. and 150.degree. C. for polypropylene; and in the range between 220.degree. and 225.degree. C. for polyester (PBTP or PETP). It is precisely these polyesters that must be regarded as synthetics that are difficult to heat seal. On the one hand there is the requirement that the package in each case should be leak-proof, and, on the other hand, intensive heat sealing must be possible.
If we want to heat seal two parts coated with synthetic materials, then one critical property is the short solidification temperature interval, i.e. a short temperature interval between the liquid and the solid state of the synthetic. When using polyester, this interval becomes so small that a good heat seal is difficult to obtain.
If too much heat is applied such that too much heat penetrates through the cardboard carrier material to the synthetic or lacquer coating, then there is often the danger of burning, this is, brown spots occur, which detrimentally affect the appearance of the finished, closed packaging. Also, the printer's ink could be damaged by being heated too much.
It is known in principle that sheets of cardboard carrier material coated with synthetic resin can be combined with one another by being welded or sealed by constant heat baking, by pressing a so-called upper heater against a counter-pressure plate, and that using this, the temperature also penetrates through the carrier material to the coating which is to be liquified. In other welding techniques, the use of a lower heater is known, and in still other methods, the use of a lower heater and an upper heater is known. Heating by application of hot air and subsequent pressing by means of cold cheeks has also proven practical for polyester.